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Specific polycyclic aromatic hydrocarbons identified as ecological risk factors in the Lagos lagoon, Nigeria
Environmental Pollution 255 (2019) 113295
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Specific polycyclic aromatic hydrocarbons identified as ecological risk factors in the Lagos lagoon, Nigeria* Temitope O. Sogbanmu a, *, Adesola O. Osibona b, Adebayo A. Otitoloju a a b
Ecotoxicology and Conservation Unit, Department of Zoology, Faculty of Science, University of Lagos, Akoka, Lagos, Nigeria Department of Marine Science, Faculty of Science, University of Lagos, Akoka, Lagos, Nigeria
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 January 2019 Received in revised form 18 September 2019 Accepted 19 September 2019 Available online 20 September 2019
The ecological risk assessment (ERA) of polycyclic aromatic hydrocarbons (PAHs) is imperative due to their ubiquity and biological effects in aquatic organisms. We evaluated the seasonal levels and ERA of 16 priority PAHs in surface water, sediment and fish from four (4) anthropogenic-impacted zones of the Lagos lagoon, Nigeria. PAHs were analysed using GC-FID and standard guidelines were utilized for the ERA. Also, we developed a sediment to water-PAHs ratio and ranking of PAHs for the ERA. The dominant fish species were Sarotherodon melanotheron (Black-Jawed Tilapia), Gerres melanopterus (Gerres), Liza falcipinnis (Sicklefin Mullet) and Pseudotolithus elongatus (Bobo Croaker) at Ilaje, Iddo, Atlas cove and Apapa zones respectively. The range of sum PAHs was 195e1006 mg L1, 302e1290 mg kg1 and 8.80 e26.1 mg kg1 in surface water, sediment and fish species respectively. Naphthalene was dominant in the surface water and sediment samples while 3-ring to 4-ring PAHs were predominant in fish species across the zones and seasons. The sediment to water-PAHs ratio was greater than 1 for sum PAHs and significantly higher (p < 0.05) in the wet season for specific PAHs across the zones and seasons. On the basis of the sediment PAHs level, Apapa zone was highly polluted with frequent biological effects while the other zones were moderately polluted with occasional biological effects across the zones and seasons. Fish species from all zones were minimally contaminated in both seasons except S. melanotheron which was not contaminated. The specific PAHs identified as ecological risk factors in the lagoon and ranking based on 50e75% recurrence in the ERA were; naphthalene, acenaphthene > acenaphthylene, fluorene, pyrene and benzo[a]anthracene. We recommend that the specific PAHs identified should form the basis for the establishment of environmental quality standards for individual PAHs in coastal waters based on the UN sustainable development goal 14 (life below water). © 2019 Elsevier Ltd. All rights reserved.
Keywords: Ecological risk assessment PAHs Sediment Surface water Fish species
1. Introduction Anthropogenic processes that promote the ubiquity of polycyclic aromatic hydrocarbons (PAHs) in aquatic ecosystems include oil spills, shipping, urban run-off, wastewater discharges as well as atmospheric fallouts of vehicle exhaust and industrial stack emissions (Qiu et al., 2009). Several studies have documented sum PAHs concentrations in surface waters from 0.04 mg L1 in Guanabara bay, Brazil (Meniconi et al., 2002) to 7510 mg L1 in Douglas creek, Niger Delta, Nigeria (Edouk et al., 2010). In sediments, sum PAHs concentrations vary from 9 mg kg1 in Naples harbour, Italy (Sprovieri
* This paper has been recommended for acceptance by Eddy Y. Zeng. * Corresponding author. E-mail address: [email protected] (T.O. Sogbanmu).
https://doi.org/10.1016/j.envpol.2019.113295 0269-7491/© 2019 Elsevier Ltd. All rights reserved.
et al., 2007) to 1943000 mg kg1 (1943 mg g1) in the Rivers of Tianjing, China (Shi et al., 2005). In fish species, total PAHs values ranging from 3.99 mg kg1 in the liver of Boleophthalmus dussumieri (mudskipper) from the Persian Gulf (Sinaei and Mashinchian, 2014) to 167 mg kg1 in Galeoides decadactylus from the Ghanaian coastal waters (Nyarko et al., 2011) have been documented. On the basis of their physical and biological properties, PAHs are classified into the low molecular weight-PAHs (LMW-PAHs) bearing 2e3 aromatic rings and the high molecular weight PAHs (HMW-PAHs) consisting 4e6 aromatic rings (ATSDR, 2000). Eight (8) of the HMW-PAHs are classified as carcinogenic PAHs (carc.PAHs) due to their known or potential carcinogenicity (IARC, 2010) (SI 1). The HMW-PAHs are not readily degraded by microorganisms and can thus persist in the aquatic environment by bioaccumulating in aquatic organisms like fish and mussels (Rocher et al., 2004). The Lagos lagoon is a wide expanse of estuarine water and the
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largest lagoon system off the Gulf of Guinea (Adeogun et al., 2019). It has received a lot of attention due to the proliferation of industries and residential houses that discharge their effluents (often untreated) into it and the increasing population in the coastal city of Lagos, Nigeria (Amaeze et al., 2015; Sogbanmu et al., 2016; Adeogun et al., 2019). The Apapa port, the busiest port in Nigeria and perhaps in West Africa has been documented to be highly contaminated due to the various activities by petroleum tank farms located along the terminal including ships and boats associated with the import and export of petroleum products (Sogbanmu et al., 2016). Sediments are important sinks of PAHs in the aquatic environment due to the hydrophobic nature of the latter and high organic matter in sediments (Stark et al., 2003). Several studies have documented the adverse effects of PAHs on fisheries and aquatic ecosystems. For instance, embryotoxic and genotoxic effects in zebrafish (Danio rerio) embryos have been observed in sediment extracts contaminated with PAHs as well as environmentally relevant concentrations of PAHs mixtures (Sogbanmu et al., 2016). Furthermore, adverse effects in fish species especially embryos exposed to toxic PAHs released in the Deepwater horizon oil spill have been observed several years after the incident (Mager et al., 2014; Esbaugh et al., 2016). Further, sexspecific biological responses have been reported in fish exposed to sublethal concentrations of PAHs (Sogbanmu et al., 2018) as well as incidence of intersex in fish species harbouring certain anthropogenic-impacted areas of the Lagos lagoon (Adeogun et al., 2019). Ecological risk indices and sediment quality guidelines have been developed based on observed adverse effects in aquatic organisms resident in polluted aquatic ecosystems. These indices such as the threshold effects levels (TELs), probable effect levels (PEL), effects range low (ERL) and effects range medium (ERM) values (Long et al., 1995; MacDonald et al., 1996) can be utilized to classify the pollution status of aquatic ecosystems for ecomonitoring and management interventions. The guidelines were developed using sediment chemistry and biological effects data for marine environments (Long et al., 1995; MacDonald et al., 1996). Another ecological risk index is the Probable Effects Level quotient (PELq). It describes the contamination effect of PAHs on biological organisms based on the analyses of chemical data and matching toxicity in 1068 sediment samples from coastal waters and estuaries in the USA (Long et al., 1995). This factor is practical for comparing different historical episodes or other study areas and to facilitate the regulator’s or policymaker’s work in sediment quality assessment (McCready et al., 2006). Consequently, the aim of this study was to assess the seasonal levels and ecological risks of sixteen (16) priority PAHs in surface water, sediment and fish species from four (4) anthropogenicimpacted zones of the Lagos lagoon, Nigeria. Also, a sediment to water-PAHs ratio was developed to establish the partitioning and specific PAHs identified as ecological risk factors in aquatic ecosystems. We aim to identify specific PAHs which are ecological risk factors in the Lagos lagoon and particularly at the study sites for targeted monitoring and intervention purposes to support the UN SDG 14 on sustaining life below water. 2. Materials and methods 2.1. Study area, sampling regime and surface water physicochemical parameters evaluation The study area was the Lagos lagoon which was subdivided into four (4) sampling zones on the basis of the kind of anthropogenic activities in the areas (SI 2). Sampling was conducted in the peak wet (July) and dry (January) seasons from 2012 to 2014.
Three (3) surface water samples from each zone were collected at depths of 0.5 cm using pre-cleaned amber glass bottles. The bottles were washed with tap water and detergent after which they were rinsed with tap water followed by acetone prior to their use for sampling (Adeniji et al., 2017). Sediments were sampled at depths of 0e10 cm with the aid of a Van Veen grab sampler while dominant fish species from each zone were caught with the assistance of fishermen using fishing nets (Olaniran et al., 2019). Sediments and fish samples were wrapped with aluminium foil separately and stored in pre-labelled glass jars placed on ice. All samples were transported on ice to the laboratory where they were stored at 20 C prior to analysis. Samples were collected as composites of three (3) sampling stations per zone for each matrix and season. The fish species were identified by a Fish Taxonomist in the Department of Marine Science, Faculty of Science, University of Lagos, Nigeria. The physicochemical parameters of the surface water were analysed in situ with the aid of a Horiba U50G multi-water sampler. The parameters analysed were pH, salinity, temperature, conductivity, dissolved oxygen and total dissolved solids. 2.2. PAHs extraction 500 mL of the water sample was transferred into a 1 L separatory funnel and 60 mL of redistilled DCM was added. The separatory funnel was shaken vigorously for 2 min with periodic venting to release vapour pressure. The organic layer was allowed to separate for 10 min and was recovered into a 250 mL flask. The aqueous layer was re-extracted twice with 60 mL of the extractant. The combined extract was dried by passing through the funnel containing anhydrous sodium sulphate. The dried extract was concentrated with a stream of nitrogen gas. 20 g of pulverized sediment or fish sample was weighed into a 250 mL capacity beaker of borosilicate material and 100 mL of redistilled hexane:DCM in ratio 3:1 was added. The beaker and its content were placed in a sonicator to extract the hydrocarbons for about 2 h. The organic layer was filtered into a 250 mL borosilicate beaker. The extract was dried by passing the filtrate through the funnel containing anhydrous sodium sulphate. The dried extract was concentrated with a stream of nitrogen gas (Sogbanmu et al., 2016). Whole fish tissue samples were homogenised, extracted and analysed for PAHs as described above following the procedures in Nwaichi and Ntorgbo (2016). Sixteen (16) priority PAHs (naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranathene, benzo[a]pyrene, indeno[1,2,3-cd]pyrene, dibenzo[a,h] anthracene and benzo[g,h,i]perylene) were extracted and analysed according to Bi et al. (2009) and Olaniran et al. (2019). 2.3. PAHs separation, clean up, instrumental analysis and quality assurance The concentrated oil was separated into the aliphatic profiles and PAHs by packing the glass column with activated alumina, neutral and activity/grade 1. 10 mL of the treated alumina was packed into the column and cleaned properly with redistilled hexane. The extract was poured onto the alumina and allowed to run down with the aid of the redistilled hexane to remove the aliphatic profiles into a pre-cleaned 20 mL glass container. The aromatic fraction was recovered by allowing the mixture of hexane and DCM in ratio 3:1 and finally removed the most polar PAHs by removing with the DCM into a pre-cleaned borosilicate beaker. The mixture was concentrated to 1 mL by a stream of nitrogen gas before the gas chromatography analysis (Sogbanmu et al., 2016).
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PAHs were analysed using a Hewlett Packard Gas chromatograph 6890 coupled to flame ionization detector and HP ChemStation Rev. A 09.01 [1206] software. Gas chromatographic conditions were; column (HP-1), column length (30 m), column ID (0.25 mm), column film (0.25 mm), injection temperature (250 C), detector temperature (320 C), initial temperature (60 C), first rate (15 C min1 for 14 min and maintained for 3 min), second rate (10 C min1 for 5 min and maintained for 4 min), mobile phase or carrier (nitrogen), nitrogen column pressure (30 psi), hydrogen pressure (28 psi) and compressed air pressure (32 psi) (Sogbanmu et al., 2016). Appropriate quality assurance precautions were taken during sampling, sample preservation and labelling. Also, analytical grade reagents were utilized for the PAHs extraction and analysis according to Bi et al. (2009) and Olaniran et al. (2019). Briefly, a reagent blank was carried out for the water sample using distilled water extracted with hexane/DCM solvent used for the liquidliquid extraction. For the fish samples, extraction efficiency and recovery of PAHs in the samples followed the protocols in Nwaichi and Ntorgbo (2016). For the sediment sample, a certified reference material was used which was ultrasonicated and extracted for PAHs like the sediment samples. Spiked recovery method was used for validation studies of the sediment samples. The samples were spiked with 1 mL of 100 ppm standard mixture containing the 16 priority PAHs before extraction. The spiked samples were extracted and analysed. The recoveries were between 75 and 105% for both the spiked and certified reference standard and certified level were obtained for PAHs in the CRM. A mixed standard of the 16 priority PAHs was used for external calibration of the instrument. Calibration standards were prepared by serial dilution of stock solution with DCM. A calibration curve was plotted for each of the PAHs. All the calibration plots for the 16 PAHs had R2 value of 0.98e0.99 and were used to quantify the PAHs.
2.4. PAHs sediment to water partitioning and bioconcentration factor (BCF) The estimation of the sediment to water-PAHs ratio in this study was to delineate the extent to which the sediments serve as a repository for PAHs, as well as, identify specific PAHs which may be determinants of this attribute. PAHs (especially HMW-PAHs which are more resistant to volatilization and biodegradation and hence highly toxic to organisms) are more concentrated in sediments than in water due to their hydrophobic nature (Keshavarzifard et al., 2014; Asagbra et al., 2015). The sediment to water-PAHs ratio could be considered as an ecological risk index due to the potential adverse biological effects that could be elicited in organisms (particularly benthic organisms) which are exposed to PAHs in sediments. These effects have been demonstrated in various studies using sediment extracts (Goswami et al., 2016; Sogbanmu et al., 2016) in which sediment PAHs levels were higher than the levels in the surface water medium. Hence, we developed an equation to show the sediment to water PAHs partitioning (that is, the relationship between sediment and surface water PAHs level) as follows:
sediment to water PAHs ¼
sediment PAHs level surface water PAHs level
If S/W-PAHs ¼ 1, then sediment PAHs is equal to surface water PAHs level, sediment to water-PAHs<1, then surface water PAHs is higher than sediment PAHs level, sediment to water-PAHs>1, then sediment PAHs is higher than surface water PAHs level. Hence, a sediment to water-PAHs ratio>1 portends a high risk of adverse
3
biological effects to exposed organisms compared to a sediment to water-PAHs ratio<1. The estimation of PAHs bioconcentrated in the tissues of the fish species sampled from the Lagos lagoon was calculated as a dimensionless factor called the BioConcentration Factor or Biota Concentration factor modified after Walker (1987). It is expressed as;
Biota Concentration Factor ðBCFÞ ¼
PAHs level in Fish PAHs level in water
2.5. Estimation of biological effects/toxicity of sediments based on four indices The potential biological effects of PAHs in the sediment on resident organisms in the lagoon were assessed by comparing with sediment toxicity screening guidelines/values. First, we evaluated this using the Effects Range Low/Effects Range Medium (ERL/ERM) and Threshold Effects Level/Probable Effects Level (TEL/PEL) values (SI 3A) (Long et al., 1995; MacDonald et al., 1996). We designated values between 0 and 1 for ease of grouping of the zones for the various ratios and corresponding effects of the ERL/ERM and TEL/ PEL (SI 3B). Secondly, we evaluated the Probable Effects Level quotient (PELq) which is the average of the ratios between the PAHs concentration in the sediment sample and the related PEL value (Khairy et al., 2009), that is, PELq ¼ average PAH level in sediment/ related PEL value. The PELq of the sediments was divided into four categories (SI 3C). Thirdly, we calculated the mean quotients for all PAHs (Long and P MacDonald, 1998) as follows: m-ERM-q ¼ (Ci/ERMi)/n. Where Ci ¼ concentration of PAH, ERMi ¼ ERM value for the same target of PAH and n ¼ the number of PAHs. m-ERM-q was categorized into four levels according to their probability of toxicity (SI 3D). 2.6. Classification of pollution based on sum polycyclic aromatic hydrocarbons in sediment and fish The sediment pollution level was classified according to Baumard et al. (1998) (SI 3E) while the level of contamination of P PAHs in the fish species sampled from the Lagos Lagoon were determined by comparing with set criteria (Soares-Gomes et al., 2010) (SI 3F). 2.7. Identification of specific polycyclic aromatic hydrocarbons as ecological risk factors in the Lagos lagoon In order to provide evidence-based data for intervention and ecosystems management by environmental regulators, policymakers and other stakeholders, we identified specific PAHs based on the various ecological risk indices and corresponding effects estimated. These PAHs were ranked (in percentages) based on the number of times identified as ecological risk factors in the four ecological risk indices which specified pollution level or biological effects of PAHs in water, sediment and fish species from the Lagos lagoon (SI 3G). 2.8. Statistical analysis All results are presented in tables as mean values. Histogram was used for graphical representation of some results. One-way analysis of variance (ANOVA) was used to test for significant differences between the levels of PAHs at the various zones, samples
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(p < 0.05) than values at the Ilaje and Iddo zones in the wet season P while the LMW-PAHs value at the Ilaje zone was significantly lower (p < 0.05) than values at the other zones in the dry season P (Table 2). The LMW-PAHs made up 44.4% (Ilaje zone, wet season) P to 85.6% (Atlas Cove zone, wet season), HMW-PAHs made up 14.4% (Atlas Cove zone, dry season) to 55.6% (Ilaje zone, wet season) P while carc.-PAHs made up 12% (Atlas Cove zone, wet season) to 45.5% (Ilaje zone, wet season) of the PAHs profile (SI 8A). The decreasing order of 2- to 6-ring PAHs was as follows; 2-ring PAHs>3-ring>4-ring PAHs>5-ring PAHs>6-ring PAHs (SI 8B).
and seasons. The level of significance was set at p < 0.05. Post-hoc tests were conducted using Duncan Multiple Range Test (Duncan, 1955). All results were analysed using SPSS version 20.0. MS Excel 2010 was used for the graphical representation of data. 3. Results 3.1. Physicochemical parameters of surface water samples from the Lagos lagoon The pH of the surface water was slightly alkaline with higher values (p > 0.05) in the dry season at all zones except Apapa (SI 4). Temperature was higher across the zones in the dry season but significantly higher (p < 0.05) at Iddo and Atlas cove zones. Dissolved oxygen (DO) was lower in the dry season across the zones except at Apapa where it was higher (p > 0.05) in the dry season. The conductivity, salinity and TDS were significantly higher (p < 0.05) in the dry season across all zones compared to the wet season (SI 4).
3.4. Polycyclic aromatic hydrocarbons in fish species of the Lagos Lagoon The dominant fish species were Sarotherodon melanotheron (Black-Jawed Tilapia), Gerres melanopterus (Gerres), Liza falcipinnis (Sicklefin Mullet), Pseudotolithus elongatus (Bobo Croaker) at the Ilaje, Iddo, Atlas Cove and Apapa zones respectively (SI 9A-D). The value of individual PAHs in the fish species ranged from 0.01 mg kg1 (Benzo[k]fluoranthene, all zones/fish species and seasons except S. melanotheron/Ilaje zone) to 8.77 mg kg1 (phenanthrene, L. falcipinnis/Atlas Cove zone, wet season). Benzo[a] anthracene, chrysene, benzo[b]fluoranthene, benzo[a]pyrene and indeno[1,2,3-cd]pyrene were not detected in some fish species/ P P zones and seasons (SI 10). The LMW-PAHs, carc.PAHs and P PAHs values in S. melanotheron/Ilaje zone were significantly lower (p < 0.05) than values in fish species from the other zones in both seasons. However, there were no significant differences in the P values of HMW-PAHs in fish species across the zones and seasons P (Table 3). The LMW-PAHs made up 30% (S. melanotheron/Ilaje zone, wet season) to 63.6% (P. elongatus/Apapa zone, dry season), P HMW-PAHs made up 36.2% (P. elongatus/Apapa zone, dry season) P to 70% (S. melanotheron/Ilaje zone, wet season) while carc.-PAHs made up 1.1% (S. melanotheron/Ilaje zone, wet season) to 10.1% (G. melanopterus/Iddo zone, wet season) of the PAHs profile (SI 11A). The decreasing order of 2- to 6-ring PAHs was as follows; 2ring PAHs>3-ring PAHs>4-ring PAHs>5-ring PAHs>6-ring PAHs (SI 11B).
3.2. Polycyclic aromatic hydrocarbons in surface water of the Lagos Lagoon The values of individual PAHs in surface water ranged from 0.11 mg L1 (fluorene, Iddo zone, wet season) to 359 mg L1 (naphP thalene, Apapa zone, dry season) (SI 5). The LMW-PAHs value at the Apapa zone was significantly higher (p < 0.05) than values at P the other zones in the wet and dry seasons. The HMW-PAHs and P carc.-PAHs values at the Atlas Cove zone were significantly higher (p < 0.05) than values at the Ilaje and Iddo zones in the wet P season. The PAHs value at the Apapa zone was significantly higher (p < 0.05) than values at the other zones in the wet season as well as the Ilaje and Atlas Cove zones in the dry season (Table 1). P The LMW-PAHs made up 47% (Atlas cove zone, wet season) to P 83% (Apapa zone, dry season), HMW-PAHs made up 17% (Apapa zone, dry season) to 53% (Atlas Cove zone, wet season) while P carc.-PAHs made up 12% (Apapa zone, dry season) to 47% (Atlas Cove zone, wet season) of the PAHs profile (SI 6A). The decreasing order of 2- to 6-ring PAHs was as follows; 2-ring PAHs>3-ring PAHs, 5-ring PAHs>4-ring PAHs>6-ring PAHs (SI 6B).
3.5. Polycyclic aromatic hydrocarbons sediment to water partitioning and bioconcentration factor in fish species
3.3. Polycyclic aromatic hydrocarbons in sediments of the Lagos Lagoon
Sediment to water PAHs ratio ranged from 0.17 to 3.19 in the wet season and 0.73 to 2.46 in the dry season (Fig. 1). The sediment to water-PAHs ratio was greater than 1 for specific PAHs namely naphthalene, acenaphthylene, anthracene (dry season only), pyrene, benzo[a]anthracene, chrysene and benzo[b]fluoranthene in both seasons. Other PAHs had values less than 1 except for indeno [1,2,3-cd]pyrene with S/W-PAHs ratio equal to 1 in the dry season
The individual PAHs in sediments of the Lagos lagoon ranged from 0.04 mg kg1 (fluorene, Iddo zone, wet season and benzo[g,h,i] perylene, Atlas Cove zone, wet season) to 623 mg kg1 (naphthaP lene, Atlas Cove zone, wet season) (SI 7). The LMW-PAHs and P PAHs values at the Apapa zone were significantly higher
Table 1 Seasonal levels of polycyclic aromatic hydrocarbons in surface water samples from the Lagos lagoon. PAHs (mg/L)
Ilaje Wet
Sum PAHs
P LMW-PAHs P HMW-PAHs P carc.PAHs P PAHs
a
116 78.6a 64.2a 195a
Iddo Dry
Wet ab
211 99.8ab 88.9a 311ab
ab
199 86.5a 74.9a 285ab
Atlas Cove Dry
Wet bc
501 114ab 104a 615abc
ab
203 233b 206b 435ab
Apapa Dry
Wet abc
421 92ab 84a 513ab
c
783 223ab 153ab 1006c
Dry 632c 131ab 90a 763bc
Note: Dissimilar letters (in superscripts) represent significant difference between treatment means at p < 0.05. n ¼ 2 (that is, years 1 and 2). P Key: LMW-PAHs (Sum 6 Low Molecular weight polycyclic aromatic hydrocarbons) ¼ Naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene and anthracene; P HMW-PAHs (Sum 10 High Molecular weight polycyclic aromatic hydrocarbons) ¼ Fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k] P fluoranthene, benzo[a]pyrene, Indeno[1,2,3-cd]pyrene, dibenzo[a,h]anthracene and benzo[g,h,i]perylene; carc.PAHs (Sum 8 Carcinogenic polycyclic aromatic hydrocarbons) ¼ benzo(a)anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, Indeno[1,2,3-cd]pyrene, dibenzo[a,h] anthracene and benzo[g,h,i] P perylene; PAHs (Sum 16 Polycyclic aromatic hydrocarbons).
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Table 2 Seasonal levels of polycyclic aromatic hydrocarbons in sediment samples from the Lagos lagoon. PAHs (mg/kg)
Ilaje Wet
Sum PAHs
P LMW-PAHs P HMW-PAHs P carc.PAHs P PAHs
a
134 168ab 137ab 302a
Iddo Dry
Atlas Cove
Wet a
227 138ab 113ab 364ab
Dry
ab
Wet bc
378 138ab 83.38a 516ab
618 174ab 129ab 792abc
bc
688 123a 97.3ab 811abc
Apapa Dry
Wet bc
715 121a 103ab 836abc
c
790 500b 346b 1290c
Dry 699bc 306ab 216ab 1005bc
Note: Dissimilar letters (in superscripts) represent significant difference between treatment means at p < 0.05. n ¼ 2 (that is, years 1 and 2). P Key: LMW-PAHs (Sum 6 Low Molecular weight polycyclic aromatic hydrocarbons) ¼ Naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene and anthracene; P HMW-PAHs (Sum 10 High Molecular weight polycyclic aromatic hydrocarbons) ¼ Fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k] P fluoranthene, benzo[a]pyrene, Indeno[1,2,3-cd]pyrene, dibenzo[a,h]anthracene and benzo[g,h,i]perylene; carc.PAHs (Sum 8 Carcinogenic polycyclic aromatic hydrocarbons) ¼ benzo(a)anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, Indeno[1,2,3-cd]pyrene, dibenzo[a,h] anthracene and benzo[g,h,i] P perylene; PAHs (Sum 16 Polycyclic aromatic hydrocarbons).
Table 3 Seasonal levels of polycyclic aromatic hydrocarbons in fish samples from the Lagos lagoon. PAHs (mg/kg)
Sum PAHs
P LMW-PAHs P HMW-PAHs P carc.PAHs P PAHs
Ilaje - Sarotherodon melanotheron
Iddo e Gerres melanopterus
Atlas Cove e Liza falcipinnis
Wet
Dry
Wet
Dry
Wet
Dry
Wet
Dry
2.67a 6.22a 0.10a 8.89a
2.77a 6.03a 0.10a 8.80a
11.6b 11.5a 2.32b 23.1b
13.2b 12.86a 2.29b 26.1b
13.1b 10.1a 2.31b 23.2b
14.3 b 10.5a 2.35b 24.7b
12.8b 7.28a 1.85b 20.1ab
11.4b 12.0a 2.35b 23.4b
Apapa - Pseudotolithus elongatus
Note: Dissimilar letters (in superscripts) represent significant difference between treatment means at p < 0.05. n ¼ 2 (that is, years 1 and 2). P Key: LMW-PAHs (Sum 6 Low Molecular weight polycyclic aromatic hydrocarbons) ¼ Naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene and anthracene; P HMW-PAHs (Sum 10 High Molecular weight polycyclic aromatic hydrocarbons) ¼ Fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k] P fluoranthene, benzo[a]pyrene, Indeno[1,2,3-cd]pyrene, dibenzo[a,h]anthracene and benzo[g,h,i]perylene; carc.PAHs (Sum 8 Carcinogenic polycyclic aromatic hydrocarbons) ¼ benzo(a)anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, Indeno[1,2,3-cd]pyrene, dibenzo[a,h] anthracene and benzo[g,h,i] P perylene; PAHs (Sum 16 Polycyclic aromatic hydrocarbons).
Fig. 1. Partitioning of polycyclic aromatic hydrocarbons in surface water and sediment from the Lagos lagoon based on sediment to water-PAHs ratio, KEY: n ¼ 4. *represents significant difference between means at p < 0.05. Naph e naphthalene, Acenaph e acenaphthene, Acenaphthy e acenaphthylene, Fluo e fluorene, Phen e phenanthrene, Pyr e pyrene, BaA e benzo[a]anthracene, Chry e chrysene, BbF e benzo[b]fluoranthene, BkF e benzo[k]fluoranthene, BaP e benzo[a]pyrene, IdP e indeno[1,2,3-cd]pyrene, DahA dibenzo[a,h] anthracene, BghiP - benzo[g,h,i]perylene.
only. However, the sediment to water-PAHs ratios for the sum PAHs were greater than 1 in both seasons (Fig. 1). Significant increases (p < 0.05) were observed for naphthalene (2.19), pyrene (3.19), benzo[a]anthracene (1.81), chrysene (1.70) and benzo[k]fluoranthene (2.48) in the wet season compared to the dry season (Fig. 1). The BCF values ranged from 0.0 to 0.50 for S. melanotheron (Ilaje zone), 0.0e2.28 for G. melanopterus (Iddo zone), 0.0e0.64 for Liza falcipinnis (Atlas Cove zone) and 0.0e0.63 for P. elongatus (Apapa
zone) (SI 12). The BCF values were less than 1 in all the fish species and PAHs except for G. melanopterus with a BCF value of 2.28 for pyrene (SI 12).
3.6. Biological effects/toxicity of sediments The comparison of the sediment PAHs concentration with ERL/ ERM values revealed that biological effects would occur occasionally in resident aquatic organisms based on the levels of
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naphthalene, acenaphthylene, acenaphthene, fluorene and sum LMW-PAHs at all zones and in both seasons especially at the Apapa zone (Fig. 2). Adverse biological effects will occur occasionally at all zones and in both seasons based on the levels of naphthalene, acenaphthylene, acenaphthene, fluorene and benzo(a)anthracene (Fig. 3) compared with the TEL/PEL values. Furthermore, adverse biological effects will occur frequently at all zones in the dry season based on the level of naphthalene and at the Apapa zone in both seasons based on the level of naphthalene, acenaphthylene and acenaphthene (Fig. 3). The estimation of the probable effects quotient/factor (PELq) showed that slightly adverse effects will occur at all zones and in both seasons except at the Ilaje zone (wet and dry seasons) and Iddo zone (wet season) (SI 13). There is an 11% probability of toxicity to organisms (m-ERMq0.1) at all the zones and in both seasons based on the evaluation of the mean-effects range medium-quotient (m-ERM-q) (SI 14).
3.7. Pollution classification of sediments and fish species Sediments from all the zones and in both seasons were moderately polluted while sediments from the Apapa zone were highly polluted in the wet season based on the pollution classification of sum polycyclic aromatic hydrocarbons in sediments (Fig. 4). Fish species from all the zones and in both seasons were minimally contaminated except for S. melanotheron from the Ilaje zone which was not contaminated in both seasons based on the contamination classification of sum PAHs in fish species (Fig. 5).
3.8. Ranking of specific polycyclic aromatic hydrocarbons identified as ecological risk factors in the Lagos lagoon The ranking of specific PAHs identified as ecological risk factors in water, sediment and fish species from the Lagos lagoon showed that naphthalene and acenaphthene occurred in Three (3) (75%) out of the Four (4) ecological risk indices (SI 3G) specifying potential biological effects of specific PAHs (Fig. 6). The second most identified PAHs were acenaphthylene, fluorene, pyrene and benzo[a] anthracene occurring twice in the ecological risk indices. The least occurring PAHs identified as ecological risk factors were anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene and indeno[1,2,3-cd]pyrene (Fig. 6).
4. Discussion In this study, the DO and TDS values across the zones and seasons were lower and higher than the set limits respectively by the National Environmental Standards and Regulations Enforcement Agency (NESREA, 2010). The range of pH and temperature recorded in this study is similar to the values observed previously in the lagoon (Ayoola and Kuton, 2009). The salinity regime of the Lagos lagoon has been linked to the insignificant pH range observed in this environment. High water salinity in the dry season has been majorly attributed to low freshwater inputs and increased evaporation and vice versa during the wet season in the lagoon (Ayoola and Kuton, 2009). The dominance of naphthalene (2-ring PAHs) in the samples across the zones is consistent with the results of Asagbra et al. (2015) in the Warri River, Nigeria and Li et al. (2015) in the Yangpu Bay, China. Similarly, Sinaei and Mashinchian (2014) observed a dominance of LMW-PAHs in the coastal waters of the Persian Gulf. High LMW-PAHs level may be due to their solubility and persistence in the water column compared to the HMW-PAHs which are more hydrophobic, thus bound to sediments (Sinaei and P Mashinchian, 2014). The range of 16 PAHs in the surface waters is several folds higher than the values reported by Benson et al. (2014). The highest PAHs level in both seasons recorded at the Apapa zone could be due to the high volume of petroleum-related activities that are characteristic of the zone which has been indicated by various researchers (Doherty and Otitoloju, 2016; Sogbanmu et al., 2016). The preponderance of petroleum tank farms around this zone, high traffic of ships and boats utilizing petroleum products are potential cursors of the observed high PAHs level. The observed high levels of HMW-PAHs which include carcinogenic PAHs in the wet season compared to the dry season may be attributable to urban run-off of oil and atmospheric deposition, as well as, resuspension of HMW-PAHs that are adsorbed onto sediments into the water column during the wet season. It P is noteworthy that the high percentage composition of carc.PAHs in the HMW-PAHs profile points to the degree of potentially mutagenic, genotoxic and carcinogenic pollutants that are present in the Lagos lagoon water column (Adeogun et al., 2019). The dominance of the 2-ring PAHs (naphthalene) in the sediments is consistent with the results of Sojinu et al. (2013) who reported a similar trend in the sediments of the Ologe lagoon. However, Karacik et al. (2009) reported a dominance of 4e5 ring PAHs in the sediments of the Istanbul strait, Turkey. Sum carcinogenic PAHs, also known as toxic PAHs (Tavakoly Sany et al., 2014) in
Fig. 2. Estimated biological effects in Lagos lagoon sediments based on the values of polycyclic aromatic hydrocarbons compared to their respective Effects Range Low/Effects Range Medium (ERL/ERM) values, KEY: n ¼ 4. *represents significant difference between means at p < 0.05.
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Fig. 3. Estimated biological effects in Lagos lagoon sediments based on the values of polycyclic aromatic hydrocarbons compared to their respective Threshold Effect and Probable Effect levels (TEL/PEL), Key: ABEF - Adverse biological effects will occur frequently; ABEO - Adverse biological effects will occur occasionally.
Fig. 4. Pollution classification of Lagos lagoon sediments based on sum polycyclic aromatic hydrocarbons, Key: HPL e High Pollution Level, MPL e Moderate Pollution Level, LPL e Low Pollution Level.
the Lagos lagoon sediments were approximately 5 times higher than the range reported for the Ologe lagoon (Sojinu et al., 2013). P The percentage composition of the carc.PAHs in the sediments of this study is notable because they have greater resistance to microbial degradation (Yun et al., 2003). The highest PAH levels recorded at the Apapa zone in both seasons is consistent with the nature of anthropogenic activities at the zone as delineated in the P surface water and previous studies. However, the average PAHs in this study is lower than the values in the sediments of the Klang P strait, Turkey (Tavakoly Sany et al., 2014). The PAHs in the sediments from all the zones except Apapa were below the guideline of 1000 ng g1 (mg kg1) suggested by Johnson et al. (2002) for the protection of estuarine fish against deleterious health effects. The dominant fish species reported in this study are consistent with the findings of Ayoola and Kuton (2009) who recorded similar fish species in the Lagos lagoon. The predominance of LMW-PAHs and HMW-PAHs specifically, the 3- and 4-ring PAHs in the fish species across the zones is slightly similar to the dominant PAHs (LMW-PAHs) in the surface water and sediments. Similarly, a
dominance of HMW-PAHs was observed in fish species from GhaP naian coastal waters (Nyarko et al., 2011). The ranges of PAHs and P carc.PAHs in this study are lower than the findings of Nyan and Muralidharan (2012) in fish species from the harbour line, Mumbai, India. The ability of fish species to bioconcentrate and metabolise PAHs may depend on factors such as the route and duration of exposure, lipid content of tissues, environmental factors, species difference, age, sex and exposure to other xenobiotics (Varanasi et al., 1987). Most of the carcinogenic PAHs not detected in the fish species in this study may be due to their depuration or biotransformation (Deb et al., 2000). In this study, the observed high sediment to water-PAHs ratio indicates that PAHs in the sediments were higher than the levels in the water with certain PAHs like naphthalene, pyrene and benzo[k] fluoranthene reaching 2e3 folds in the sediment compared to the surface water in the wet season. This may be due to frequent episodes of accidental spillages at oil depots, operational shipping loss in the harbour, oil spills or leakages from ships and boats that traverse the lagoon, dredging activities in the lagoon and
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Fig. 5. Pollution classification of Lagos lagoon fish species based on sum polycyclic aromatic hydrocarbons. Key: MiC e Minimally Contaminated, NC e Not Contaminated.
Fig. 6. Ranking of specific polycyclic aromatic hydrocarbons identified as ecological risk factors in the Lagos lagoon, Key: ERI e ecological risk indices, Naph e naphthalene, Acenaph e acenaphthene, Acenaphthy e acenaphthylene, Fluo e fluorene, Pyr e pyrene, BaA e benzo[a]anthracene, An e anthracene, Chry e chrysene, BbF e benzo[b]fluoranthene, BkF e benzo[k]fluoranthene, IdP e indeno[1,2,3-cd]pyrene.
contributions from effluents that are discharged into the lagoon (Adeogun et al., 2019). The significantly high sediment to waterPAHs ratio for pyrene especially in the wet season reflects the contamination of the sediments with HMW-PAHs. Pyrene represents a major portion of the total PAHs found in contaminated sites (Luo et al., 2006) and has been used as an indicator of PAHs contamination in the monitoring of wastes (Dissanayake and Bamber, 2010). The wet season with its attendant run-off of storm water, atmospheric deposition into the water column is implicated for the deposition of this PAH into the sediments. The observation of the highest BCF of 2.28 in G. melanopterus associated with pyrene is comparable with the highest sediment to waterPAHs ratio observed for pyrene. The lipophilic nature of pyrene as a HMW-PAH, its ubiquity and persistence could promote its bioaccessibility and consequent bioavailability in the fish tissues. In this study, the Iddo zone was not as contaminated with PAHs as observed for Atlas Cove and Apapa zones. Thus, the high BCF factor for pyrene could be related to the specific nature of the dominant
fish at the zone which is different from the dominant species at the other zones. Conversely, the low BCF (<1) for the fish species from other zones and PAHs could be an indication of a minimal contamination level of PAHs in the fish species. The estimation of frequent biological effects in organisms from the Apapa zone based on the TEL/PEL values is consistent with findings of Sogbanmu et al. (2016) who reported significant teratogenic and genotoxic effects in Danio rerio exposed to sediment extracts from the zone. It is noteworthy that the specific PAHs identified as the cursors of potential biological effects in the sediments are mainly LMW-PAHs that are known to be acutely toxic (Asagbra et al., 2015) and have been identified as ecological risk factors in sediments from the Yellow River, China (Feng et al., 2016). Occasional biological effects estimated for organisms at the Atlas Cove zone may be due to the sand dredging and petroleum discharge activities peculiar to the zone. The biological effects to organisms established based on the ecological risk indices in this study corroborate observations by various authors (Amaeze et al.,
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2015; Sogbanmu et al., 2016; Adeogun et al., 2019). Although, slight adverse effects and 11% probability of toxicity was estimated for organisms across the zones based on the PELq and m-ERM-q, the potential ecological risks posed by the sediments vis-a-vis the weight of evidence shown in the PAH levels, nature of anthropogenic activities and report of previous studies points to the need to take decisive actions in order to protect and conserve the aquatic organisms and man who are consumers of fisheries from this coastal ecosystem. The classification of sediments as moderately polluted, particularly the Apapa zone which was classified as highly polluted is similar to findings in coastal sediments of China (Li et al., 2007) and Mason Bay, Korea (Yim et al., 2005). Furthermore, this result corroborates the estimation of toxicity of sediments in this study which identified frequent biological effects at the Apapa zone and occasional biological effects at the other zones and in both seasons. The minimal contamination of fish species in this study is consistent with the low bioconcentration factor values (less than 1) observed in the fish species. This may be attributed to the low lipid content of the scaled fish species (Abdel-Shafy and Mansour, 2016). Fishes tend to have lower PAH concentrations in their body tissues compared to the water in which they live due to their ability to metabolise and excrete the compounds (Johnson et al., 2002; Johnson-Restrepo et al., 2008). However, S. melanotheron at the Ilaje zone was classified as not contaminated which could be attributed to the low levels of PAHs observed at the zone. The PAHs identified as ecological risk factors in the Lagos lagoon especially naphthalene and acenaphthene are associated with various industrial activities apart from petroleum-related activities. Acenaphthene is utilized in the production of pigments, dyes, pesticides, pharmaceuticals among other uses (Abdel-Shafy and Mansour, 2016). The various industries and residential houses which discharge their effluents (either treated or untreated) into the lagoon might be relevant sources of this PAHs in the lagoon. It is noteworthy that the PAHs are risk factors in both wet and dry seasons except in some instances where specific zones and seasons are identified (SI 3G).
5. Conclusions This study has delineated the levels of PAHs in three matrices of four anthropogenic-impacted zones of the Lagos lagoon. The high PAHs content and risk to aquatic organisms based on the ecological risk indices at the Apapa zone are reminiscent of the nature of activities (mainly shipping and port activities) at the zone which are point sources of hydrocarbons input into the lagoon. A robust ecological risk assessment has been done perhaps for the first time in the Lagos lagoon which revealed the moderate to high pollution level of the sediments in particular. Specific PAHs such as naphthalene, acenaphthene, acenaphthylene, fluorene, pyrene and benzo[a]anthracene have been identified to pose major ecological risks to organisms in lagoon. We recommend their inclusion in ecomonitoring programmes based on the UN sustainable development goal 14 (life below water). Also, the ecological risk assessment studies have revealed the need for a review and/or setting of guidelines for individual and sum PAHs in coastal waters, sediments and fish for sustainability of aquatic ecosystems and safeguard of public health through fish consumption.
Conflicts of interest The authors declare no conflicts of interest.
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Specific polycyclic aromatic hydrocarbons identified as ecological risk factors in the Lagos lagoon, Nigeria* Temitope O. Sogbanmu a, *, Adesola O. Osibona b, Adebayo A. Otitoloju a a b
Ecotoxicology and Conservation Unit, Department of Zoology, Faculty of Science, University of Lagos, Akoka, Lagos, Nigeria Department of Marine Science, Faculty of Science, University of Lagos, Akoka, Lagos, Nigeria
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 January 2019 Received in revised form 18 September 2019 Accepted 19 September 2019 Available online 20 September 2019
The ecological risk assessment (ERA) of polycyclic aromatic hydrocarbons (PAHs) is imperative due to their ubiquity and biological effects in aquatic organisms. We evaluated the seasonal levels and ERA of 16 priority PAHs in surface water, sediment and fish from four (4) anthropogenic-impacted zones of the Lagos lagoon, Nigeria. PAHs were analysed using GC-FID and standard guidelines were utilized for the ERA. Also, we developed a sediment to water-PAHs ratio and ranking of PAHs for the ERA. The dominant fish species were Sarotherodon melanotheron (Black-Jawed Tilapia), Gerres melanopterus (Gerres), Liza falcipinnis (Sicklefin Mullet) and Pseudotolithus elongatus (Bobo Croaker) at Ilaje, Iddo, Atlas cove and Apapa zones respectively. The range of sum PAHs was 195e1006 mg L1, 302e1290 mg kg1 and 8.80 e26.1 mg kg1 in surface water, sediment and fish species respectively. Naphthalene was dominant in the surface water and sediment samples while 3-ring to 4-ring PAHs were predominant in fish species across the zones and seasons. The sediment to water-PAHs ratio was greater than 1 for sum PAHs and significantly higher (p < 0.05) in the wet season for specific PAHs across the zones and seasons. On the basis of the sediment PAHs level, Apapa zone was highly polluted with frequent biological effects while the other zones were moderately polluted with occasional biological effects across the zones and seasons. Fish species from all zones were minimally contaminated in both seasons except S. melanotheron which was not contaminated. The specific PAHs identified as ecological risk factors in the lagoon and ranking based on 50e75% recurrence in the ERA were; naphthalene, acenaphthene > acenaphthylene, fluorene, pyrene and benzo[a]anthracene. We recommend that the specific PAHs identified should form the basis for the establishment of environmental quality standards for individual PAHs in coastal waters based on the UN sustainable development goal 14 (life below water). © 2019 Elsevier Ltd. All rights reserved.
Keywords: Ecological risk assessment PAHs Sediment Surface water Fish species
1. Introduction Anthropogenic processes that promote the ubiquity of polycyclic aromatic hydrocarbons (PAHs) in aquatic ecosystems include oil spills, shipping, urban run-off, wastewater discharges as well as atmospheric fallouts of vehicle exhaust and industrial stack emissions (Qiu et al., 2009). Several studies have documented sum PAHs concentrations in surface waters from 0.04 mg L1 in Guanabara bay, Brazil (Meniconi et al., 2002) to 7510 mg L1 in Douglas creek, Niger Delta, Nigeria (Edouk et al., 2010). In sediments, sum PAHs concentrations vary from 9 mg kg1 in Naples harbour, Italy (Sprovieri
* This paper has been recommended for acceptance by Eddy Y. Zeng. * Corresponding author. E-mail address: [email protected] (T.O. Sogbanmu).
https://doi.org/10.1016/j.envpol.2019.113295 0269-7491/© 2019 Elsevier Ltd. All rights reserved.
et al., 2007) to 1943000 mg kg1 (1943 mg g1) in the Rivers of Tianjing, China (Shi et al., 2005). In fish species, total PAHs values ranging from 3.99 mg kg1 in the liver of Boleophthalmus dussumieri (mudskipper) from the Persian Gulf (Sinaei and Mashinchian, 2014) to 167 mg kg1 in Galeoides decadactylus from the Ghanaian coastal waters (Nyarko et al., 2011) have been documented. On the basis of their physical and biological properties, PAHs are classified into the low molecular weight-PAHs (LMW-PAHs) bearing 2e3 aromatic rings and the high molecular weight PAHs (HMW-PAHs) consisting 4e6 aromatic rings (ATSDR, 2000). Eight (8) of the HMW-PAHs are classified as carcinogenic PAHs (carc.PAHs) due to their known or potential carcinogenicity (IARC, 2010) (SI 1). The HMW-PAHs are not readily degraded by microorganisms and can thus persist in the aquatic environment by bioaccumulating in aquatic organisms like fish and mussels (Rocher et al., 2004). The Lagos lagoon is a wide expanse of estuarine water and the
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largest lagoon system off the Gulf of Guinea (Adeogun et al., 2019). It has received a lot of attention due to the proliferation of industries and residential houses that discharge their effluents (often untreated) into it and the increasing population in the coastal city of Lagos, Nigeria (Amaeze et al., 2015; Sogbanmu et al., 2016; Adeogun et al., 2019). The Apapa port, the busiest port in Nigeria and perhaps in West Africa has been documented to be highly contaminated due to the various activities by petroleum tank farms located along the terminal including ships and boats associated with the import and export of petroleum products (Sogbanmu et al., 2016). Sediments are important sinks of PAHs in the aquatic environment due to the hydrophobic nature of the latter and high organic matter in sediments (Stark et al., 2003). Several studies have documented the adverse effects of PAHs on fisheries and aquatic ecosystems. For instance, embryotoxic and genotoxic effects in zebrafish (Danio rerio) embryos have been observed in sediment extracts contaminated with PAHs as well as environmentally relevant concentrations of PAHs mixtures (Sogbanmu et al., 2016). Furthermore, adverse effects in fish species especially embryos exposed to toxic PAHs released in the Deepwater horizon oil spill have been observed several years after the incident (Mager et al., 2014; Esbaugh et al., 2016). Further, sexspecific biological responses have been reported in fish exposed to sublethal concentrations of PAHs (Sogbanmu et al., 2018) as well as incidence of intersex in fish species harbouring certain anthropogenic-impacted areas of the Lagos lagoon (Adeogun et al., 2019). Ecological risk indices and sediment quality guidelines have been developed based on observed adverse effects in aquatic organisms resident in polluted aquatic ecosystems. These indices such as the threshold effects levels (TELs), probable effect levels (PEL), effects range low (ERL) and effects range medium (ERM) values (Long et al., 1995; MacDonald et al., 1996) can be utilized to classify the pollution status of aquatic ecosystems for ecomonitoring and management interventions. The guidelines were developed using sediment chemistry and biological effects data for marine environments (Long et al., 1995; MacDonald et al., 1996). Another ecological risk index is the Probable Effects Level quotient (PELq). It describes the contamination effect of PAHs on biological organisms based on the analyses of chemical data and matching toxicity in 1068 sediment samples from coastal waters and estuaries in the USA (Long et al., 1995). This factor is practical for comparing different historical episodes or other study areas and to facilitate the regulator’s or policymaker’s work in sediment quality assessment (McCready et al., 2006). Consequently, the aim of this study was to assess the seasonal levels and ecological risks of sixteen (16) priority PAHs in surface water, sediment and fish species from four (4) anthropogenicimpacted zones of the Lagos lagoon, Nigeria. Also, a sediment to water-PAHs ratio was developed to establish the partitioning and specific PAHs identified as ecological risk factors in aquatic ecosystems. We aim to identify specific PAHs which are ecological risk factors in the Lagos lagoon and particularly at the study sites for targeted monitoring and intervention purposes to support the UN SDG 14 on sustaining life below water. 2. Materials and methods 2.1. Study area, sampling regime and surface water physicochemical parameters evaluation The study area was the Lagos lagoon which was subdivided into four (4) sampling zones on the basis of the kind of anthropogenic activities in the areas (SI 2). Sampling was conducted in the peak wet (July) and dry (January) seasons from 2012 to 2014.
Three (3) surface water samples from each zone were collected at depths of 0.5 cm using pre-cleaned amber glass bottles. The bottles were washed with tap water and detergent after which they were rinsed with tap water followed by acetone prior to their use for sampling (Adeniji et al., 2017). Sediments were sampled at depths of 0e10 cm with the aid of a Van Veen grab sampler while dominant fish species from each zone were caught with the assistance of fishermen using fishing nets (Olaniran et al., 2019). Sediments and fish samples were wrapped with aluminium foil separately and stored in pre-labelled glass jars placed on ice. All samples were transported on ice to the laboratory where they were stored at 20 C prior to analysis. Samples were collected as composites of three (3) sampling stations per zone for each matrix and season. The fish species were identified by a Fish Taxonomist in the Department of Marine Science, Faculty of Science, University of Lagos, Nigeria. The physicochemical parameters of the surface water were analysed in situ with the aid of a Horiba U50G multi-water sampler. The parameters analysed were pH, salinity, temperature, conductivity, dissolved oxygen and total dissolved solids. 2.2. PAHs extraction 500 mL of the water sample was transferred into a 1 L separatory funnel and 60 mL of redistilled DCM was added. The separatory funnel was shaken vigorously for 2 min with periodic venting to release vapour pressure. The organic layer was allowed to separate for 10 min and was recovered into a 250 mL flask. The aqueous layer was re-extracted twice with 60 mL of the extractant. The combined extract was dried by passing through the funnel containing anhydrous sodium sulphate. The dried extract was concentrated with a stream of nitrogen gas. 20 g of pulverized sediment or fish sample was weighed into a 250 mL capacity beaker of borosilicate material and 100 mL of redistilled hexane:DCM in ratio 3:1 was added. The beaker and its content were placed in a sonicator to extract the hydrocarbons for about 2 h. The organic layer was filtered into a 250 mL borosilicate beaker. The extract was dried by passing the filtrate through the funnel containing anhydrous sodium sulphate. The dried extract was concentrated with a stream of nitrogen gas (Sogbanmu et al., 2016). Whole fish tissue samples were homogenised, extracted and analysed for PAHs as described above following the procedures in Nwaichi and Ntorgbo (2016). Sixteen (16) priority PAHs (naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranathene, benzo[a]pyrene, indeno[1,2,3-cd]pyrene, dibenzo[a,h] anthracene and benzo[g,h,i]perylene) were extracted and analysed according to Bi et al. (2009) and Olaniran et al. (2019). 2.3. PAHs separation, clean up, instrumental analysis and quality assurance The concentrated oil was separated into the aliphatic profiles and PAHs by packing the glass column with activated alumina, neutral and activity/grade 1. 10 mL of the treated alumina was packed into the column and cleaned properly with redistilled hexane. The extract was poured onto the alumina and allowed to run down with the aid of the redistilled hexane to remove the aliphatic profiles into a pre-cleaned 20 mL glass container. The aromatic fraction was recovered by allowing the mixture of hexane and DCM in ratio 3:1 and finally removed the most polar PAHs by removing with the DCM into a pre-cleaned borosilicate beaker. The mixture was concentrated to 1 mL by a stream of nitrogen gas before the gas chromatography analysis (Sogbanmu et al., 2016).
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PAHs were analysed using a Hewlett Packard Gas chromatograph 6890 coupled to flame ionization detector and HP ChemStation Rev. A 09.01 [1206] software. Gas chromatographic conditions were; column (HP-1), column length (30 m), column ID (0.25 mm), column film (0.25 mm), injection temperature (250 C), detector temperature (320 C), initial temperature (60 C), first rate (15 C min1 for 14 min and maintained for 3 min), second rate (10 C min1 for 5 min and maintained for 4 min), mobile phase or carrier (nitrogen), nitrogen column pressure (30 psi), hydrogen pressure (28 psi) and compressed air pressure (32 psi) (Sogbanmu et al., 2016). Appropriate quality assurance precautions were taken during sampling, sample preservation and labelling. Also, analytical grade reagents were utilized for the PAHs extraction and analysis according to Bi et al. (2009) and Olaniran et al. (2019). Briefly, a reagent blank was carried out for the water sample using distilled water extracted with hexane/DCM solvent used for the liquidliquid extraction. For the fish samples, extraction efficiency and recovery of PAHs in the samples followed the protocols in Nwaichi and Ntorgbo (2016). For the sediment sample, a certified reference material was used which was ultrasonicated and extracted for PAHs like the sediment samples. Spiked recovery method was used for validation studies of the sediment samples. The samples were spiked with 1 mL of 100 ppm standard mixture containing the 16 priority PAHs before extraction. The spiked samples were extracted and analysed. The recoveries were between 75 and 105% for both the spiked and certified reference standard and certified level were obtained for PAHs in the CRM. A mixed standard of the 16 priority PAHs was used for external calibration of the instrument. Calibration standards were prepared by serial dilution of stock solution with DCM. A calibration curve was plotted for each of the PAHs. All the calibration plots for the 16 PAHs had R2 value of 0.98e0.99 and were used to quantify the PAHs.
2.4. PAHs sediment to water partitioning and bioconcentration factor (BCF) The estimation of the sediment to water-PAHs ratio in this study was to delineate the extent to which the sediments serve as a repository for PAHs, as well as, identify specific PAHs which may be determinants of this attribute. PAHs (especially HMW-PAHs which are more resistant to volatilization and biodegradation and hence highly toxic to organisms) are more concentrated in sediments than in water due to their hydrophobic nature (Keshavarzifard et al., 2014; Asagbra et al., 2015). The sediment to water-PAHs ratio could be considered as an ecological risk index due to the potential adverse biological effects that could be elicited in organisms (particularly benthic organisms) which are exposed to PAHs in sediments. These effects have been demonstrated in various studies using sediment extracts (Goswami et al., 2016; Sogbanmu et al., 2016) in which sediment PAHs levels were higher than the levels in the surface water medium. Hence, we developed an equation to show the sediment to water PAHs partitioning (that is, the relationship between sediment and surface water PAHs level) as follows:
sediment to water PAHs ¼
sediment PAHs level surface water PAHs level
If S/W-PAHs ¼ 1, then sediment PAHs is equal to surface water PAHs level, sediment to water-PAHs<1, then surface water PAHs is higher than sediment PAHs level, sediment to water-PAHs>1, then sediment PAHs is higher than surface water PAHs level. Hence, a sediment to water-PAHs ratio>1 portends a high risk of adverse
3
biological effects to exposed organisms compared to a sediment to water-PAHs ratio<1. The estimation of PAHs bioconcentrated in the tissues of the fish species sampled from the Lagos lagoon was calculated as a dimensionless factor called the BioConcentration Factor or Biota Concentration factor modified after Walker (1987). It is expressed as;
Biota Concentration Factor ðBCFÞ ¼
PAHs level in Fish PAHs level in water
2.5. Estimation of biological effects/toxicity of sediments based on four indices The potential biological effects of PAHs in the sediment on resident organisms in the lagoon were assessed by comparing with sediment toxicity screening guidelines/values. First, we evaluated this using the Effects Range Low/Effects Range Medium (ERL/ERM) and Threshold Effects Level/Probable Effects Level (TEL/PEL) values (SI 3A) (Long et al., 1995; MacDonald et al., 1996). We designated values between 0 and 1 for ease of grouping of the zones for the various ratios and corresponding effects of the ERL/ERM and TEL/ PEL (SI 3B). Secondly, we evaluated the Probable Effects Level quotient (PELq) which is the average of the ratios between the PAHs concentration in the sediment sample and the related PEL value (Khairy et al., 2009), that is, PELq ¼ average PAH level in sediment/ related PEL value. The PELq of the sediments was divided into four categories (SI 3C). Thirdly, we calculated the mean quotients for all PAHs (Long and P MacDonald, 1998) as follows: m-ERM-q ¼ (Ci/ERMi)/n. Where Ci ¼ concentration of PAH, ERMi ¼ ERM value for the same target of PAH and n ¼ the number of PAHs. m-ERM-q was categorized into four levels according to their probability of toxicity (SI 3D). 2.6. Classification of pollution based on sum polycyclic aromatic hydrocarbons in sediment and fish The sediment pollution level was classified according to Baumard et al. (1998) (SI 3E) while the level of contamination of P PAHs in the fish species sampled from the Lagos Lagoon were determined by comparing with set criteria (Soares-Gomes et al., 2010) (SI 3F). 2.7. Identification of specific polycyclic aromatic hydrocarbons as ecological risk factors in the Lagos lagoon In order to provide evidence-based data for intervention and ecosystems management by environmental regulators, policymakers and other stakeholders, we identified specific PAHs based on the various ecological risk indices and corresponding effects estimated. These PAHs were ranked (in percentages) based on the number of times identified as ecological risk factors in the four ecological risk indices which specified pollution level or biological effects of PAHs in water, sediment and fish species from the Lagos lagoon (SI 3G). 2.8. Statistical analysis All results are presented in tables as mean values. Histogram was used for graphical representation of some results. One-way analysis of variance (ANOVA) was used to test for significant differences between the levels of PAHs at the various zones, samples
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(p < 0.05) than values at the Ilaje and Iddo zones in the wet season P while the LMW-PAHs value at the Ilaje zone was significantly lower (p < 0.05) than values at the other zones in the dry season P (Table 2). The LMW-PAHs made up 44.4% (Ilaje zone, wet season) P to 85.6% (Atlas Cove zone, wet season), HMW-PAHs made up 14.4% (Atlas Cove zone, dry season) to 55.6% (Ilaje zone, wet season) P while carc.-PAHs made up 12% (Atlas Cove zone, wet season) to 45.5% (Ilaje zone, wet season) of the PAHs profile (SI 8A). The decreasing order of 2- to 6-ring PAHs was as follows; 2-ring PAHs>3-ring>4-ring PAHs>5-ring PAHs>6-ring PAHs (SI 8B).
and seasons. The level of significance was set at p < 0.05. Post-hoc tests were conducted using Duncan Multiple Range Test (Duncan, 1955). All results were analysed using SPSS version 20.0. MS Excel 2010 was used for the graphical representation of data. 3. Results 3.1. Physicochemical parameters of surface water samples from the Lagos lagoon The pH of the surface water was slightly alkaline with higher values (p > 0.05) in the dry season at all zones except Apapa (SI 4). Temperature was higher across the zones in the dry season but significantly higher (p < 0.05) at Iddo and Atlas cove zones. Dissolved oxygen (DO) was lower in the dry season across the zones except at Apapa where it was higher (p > 0.05) in the dry season. The conductivity, salinity and TDS were significantly higher (p < 0.05) in the dry season across all zones compared to the wet season (SI 4).
3.4. Polycyclic aromatic hydrocarbons in fish species of the Lagos Lagoon The dominant fish species were Sarotherodon melanotheron (Black-Jawed Tilapia), Gerres melanopterus (Gerres), Liza falcipinnis (Sicklefin Mullet), Pseudotolithus elongatus (Bobo Croaker) at the Ilaje, Iddo, Atlas Cove and Apapa zones respectively (SI 9A-D). The value of individual PAHs in the fish species ranged from 0.01 mg kg1 (Benzo[k]fluoranthene, all zones/fish species and seasons except S. melanotheron/Ilaje zone) to 8.77 mg kg1 (phenanthrene, L. falcipinnis/Atlas Cove zone, wet season). Benzo[a] anthracene, chrysene, benzo[b]fluoranthene, benzo[a]pyrene and indeno[1,2,3-cd]pyrene were not detected in some fish species/ P P zones and seasons (SI 10). The LMW-PAHs, carc.PAHs and P PAHs values in S. melanotheron/Ilaje zone were significantly lower (p < 0.05) than values in fish species from the other zones in both seasons. However, there were no significant differences in the P values of HMW-PAHs in fish species across the zones and seasons P (Table 3). The LMW-PAHs made up 30% (S. melanotheron/Ilaje zone, wet season) to 63.6% (P. elongatus/Apapa zone, dry season), P HMW-PAHs made up 36.2% (P. elongatus/Apapa zone, dry season) P to 70% (S. melanotheron/Ilaje zone, wet season) while carc.-PAHs made up 1.1% (S. melanotheron/Ilaje zone, wet season) to 10.1% (G. melanopterus/Iddo zone, wet season) of the PAHs profile (SI 11A). The decreasing order of 2- to 6-ring PAHs was as follows; 2ring PAHs>3-ring PAHs>4-ring PAHs>5-ring PAHs>6-ring PAHs (SI 11B).
3.2. Polycyclic aromatic hydrocarbons in surface water of the Lagos Lagoon The values of individual PAHs in surface water ranged from 0.11 mg L1 (fluorene, Iddo zone, wet season) to 359 mg L1 (naphP thalene, Apapa zone, dry season) (SI 5). The LMW-PAHs value at the Apapa zone was significantly higher (p < 0.05) than values at P the other zones in the wet and dry seasons. The HMW-PAHs and P carc.-PAHs values at the Atlas Cove zone were significantly higher (p < 0.05) than values at the Ilaje and Iddo zones in the wet P season. The PAHs value at the Apapa zone was significantly higher (p < 0.05) than values at the other zones in the wet season as well as the Ilaje and Atlas Cove zones in the dry season (Table 1). P The LMW-PAHs made up 47% (Atlas cove zone, wet season) to P 83% (Apapa zone, dry season), HMW-PAHs made up 17% (Apapa zone, dry season) to 53% (Atlas Cove zone, wet season) while P carc.-PAHs made up 12% (Apapa zone, dry season) to 47% (Atlas Cove zone, wet season) of the PAHs profile (SI 6A). The decreasing order of 2- to 6-ring PAHs was as follows; 2-ring PAHs>3-ring PAHs, 5-ring PAHs>4-ring PAHs>6-ring PAHs (SI 6B).
3.5. Polycyclic aromatic hydrocarbons sediment to water partitioning and bioconcentration factor in fish species
3.3. Polycyclic aromatic hydrocarbons in sediments of the Lagos Lagoon
Sediment to water PAHs ratio ranged from 0.17 to 3.19 in the wet season and 0.73 to 2.46 in the dry season (Fig. 1). The sediment to water-PAHs ratio was greater than 1 for specific PAHs namely naphthalene, acenaphthylene, anthracene (dry season only), pyrene, benzo[a]anthracene, chrysene and benzo[b]fluoranthene in both seasons. Other PAHs had values less than 1 except for indeno [1,2,3-cd]pyrene with S/W-PAHs ratio equal to 1 in the dry season
The individual PAHs in sediments of the Lagos lagoon ranged from 0.04 mg kg1 (fluorene, Iddo zone, wet season and benzo[g,h,i] perylene, Atlas Cove zone, wet season) to 623 mg kg1 (naphthaP lene, Atlas Cove zone, wet season) (SI 7). The LMW-PAHs and P PAHs values at the Apapa zone were significantly higher
Table 1 Seasonal levels of polycyclic aromatic hydrocarbons in surface water samples from the Lagos lagoon. PAHs (mg/L)
Ilaje Wet
Sum PAHs
P LMW-PAHs P HMW-PAHs P carc.PAHs P PAHs
a
116 78.6a 64.2a 195a
Iddo Dry
Wet ab
211 99.8ab 88.9a 311ab
ab
199 86.5a 74.9a 285ab
Atlas Cove Dry
Wet bc
501 114ab 104a 615abc
ab
203 233b 206b 435ab
Apapa Dry
Wet abc
421 92ab 84a 513ab
c
783 223ab 153ab 1006c
Dry 632c 131ab 90a 763bc
Note: Dissimilar letters (in superscripts) represent significant difference between treatment means at p < 0.05. n ¼ 2 (that is, years 1 and 2). P Key: LMW-PAHs (Sum 6 Low Molecular weight polycyclic aromatic hydrocarbons) ¼ Naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene and anthracene; P HMW-PAHs (Sum 10 High Molecular weight polycyclic aromatic hydrocarbons) ¼ Fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k] P fluoranthene, benzo[a]pyrene, Indeno[1,2,3-cd]pyrene, dibenzo[a,h]anthracene and benzo[g,h,i]perylene; carc.PAHs (Sum 8 Carcinogenic polycyclic aromatic hydrocarbons) ¼ benzo(a)anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, Indeno[1,2,3-cd]pyrene, dibenzo[a,h] anthracene and benzo[g,h,i] P perylene; PAHs (Sum 16 Polycyclic aromatic hydrocarbons).
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Table 2 Seasonal levels of polycyclic aromatic hydrocarbons in sediment samples from the Lagos lagoon. PAHs (mg/kg)
Ilaje Wet
Sum PAHs
P LMW-PAHs P HMW-PAHs P carc.PAHs P PAHs
a
134 168ab 137ab 302a
Iddo Dry
Atlas Cove
Wet a
227 138ab 113ab 364ab
Dry
ab
Wet bc
378 138ab 83.38a 516ab
618 174ab 129ab 792abc
bc
688 123a 97.3ab 811abc
Apapa Dry
Wet bc
715 121a 103ab 836abc
c
790 500b 346b 1290c
Dry 699bc 306ab 216ab 1005bc
Note: Dissimilar letters (in superscripts) represent significant difference between treatment means at p < 0.05. n ¼ 2 (that is, years 1 and 2). P Key: LMW-PAHs (Sum 6 Low Molecular weight polycyclic aromatic hydrocarbons) ¼ Naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene and anthracene; P HMW-PAHs (Sum 10 High Molecular weight polycyclic aromatic hydrocarbons) ¼ Fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k] P fluoranthene, benzo[a]pyrene, Indeno[1,2,3-cd]pyrene, dibenzo[a,h]anthracene and benzo[g,h,i]perylene; carc.PAHs (Sum 8 Carcinogenic polycyclic aromatic hydrocarbons) ¼ benzo(a)anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, Indeno[1,2,3-cd]pyrene, dibenzo[a,h] anthracene and benzo[g,h,i] P perylene; PAHs (Sum 16 Polycyclic aromatic hydrocarbons).
Table 3 Seasonal levels of polycyclic aromatic hydrocarbons in fish samples from the Lagos lagoon. PAHs (mg/kg)
Sum PAHs
P LMW-PAHs P HMW-PAHs P carc.PAHs P PAHs
Ilaje - Sarotherodon melanotheron
Iddo e Gerres melanopterus
Atlas Cove e Liza falcipinnis
Wet
Dry
Wet
Dry
Wet
Dry
Wet
Dry
2.67a 6.22a 0.10a 8.89a
2.77a 6.03a 0.10a 8.80a
11.6b 11.5a 2.32b 23.1b
13.2b 12.86a 2.29b 26.1b
13.1b 10.1a 2.31b 23.2b
14.3 b 10.5a 2.35b 24.7b
12.8b 7.28a 1.85b 20.1ab
11.4b 12.0a 2.35b 23.4b
Apapa - Pseudotolithus elongatus
Note: Dissimilar letters (in superscripts) represent significant difference between treatment means at p < 0.05. n ¼ 2 (that is, years 1 and 2). P Key: LMW-PAHs (Sum 6 Low Molecular weight polycyclic aromatic hydrocarbons) ¼ Naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene and anthracene; P HMW-PAHs (Sum 10 High Molecular weight polycyclic aromatic hydrocarbons) ¼ Fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k] P fluoranthene, benzo[a]pyrene, Indeno[1,2,3-cd]pyrene, dibenzo[a,h]anthracene and benzo[g,h,i]perylene; carc.PAHs (Sum 8 Carcinogenic polycyclic aromatic hydrocarbons) ¼ benzo(a)anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, Indeno[1,2,3-cd]pyrene, dibenzo[a,h] anthracene and benzo[g,h,i] P perylene; PAHs (Sum 16 Polycyclic aromatic hydrocarbons).
Fig. 1. Partitioning of polycyclic aromatic hydrocarbons in surface water and sediment from the Lagos lagoon based on sediment to water-PAHs ratio, KEY: n ¼ 4. *represents significant difference between means at p < 0.05. Naph e naphthalene, Acenaph e acenaphthene, Acenaphthy e acenaphthylene, Fluo e fluorene, Phen e phenanthrene, Pyr e pyrene, BaA e benzo[a]anthracene, Chry e chrysene, BbF e benzo[b]fluoranthene, BkF e benzo[k]fluoranthene, BaP e benzo[a]pyrene, IdP e indeno[1,2,3-cd]pyrene, DahA dibenzo[a,h] anthracene, BghiP - benzo[g,h,i]perylene.
only. However, the sediment to water-PAHs ratios for the sum PAHs were greater than 1 in both seasons (Fig. 1). Significant increases (p < 0.05) were observed for naphthalene (2.19), pyrene (3.19), benzo[a]anthracene (1.81), chrysene (1.70) and benzo[k]fluoranthene (2.48) in the wet season compared to the dry season (Fig. 1). The BCF values ranged from 0.0 to 0.50 for S. melanotheron (Ilaje zone), 0.0e2.28 for G. melanopterus (Iddo zone), 0.0e0.64 for Liza falcipinnis (Atlas Cove zone) and 0.0e0.63 for P. elongatus (Apapa
zone) (SI 12). The BCF values were less than 1 in all the fish species and PAHs except for G. melanopterus with a BCF value of 2.28 for pyrene (SI 12).
3.6. Biological effects/toxicity of sediments The comparison of the sediment PAHs concentration with ERL/ ERM values revealed that biological effects would occur occasionally in resident aquatic organisms based on the levels of
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naphthalene, acenaphthylene, acenaphthene, fluorene and sum LMW-PAHs at all zones and in both seasons especially at the Apapa zone (Fig. 2). Adverse biological effects will occur occasionally at all zones and in both seasons based on the levels of naphthalene, acenaphthylene, acenaphthene, fluorene and benzo(a)anthracene (Fig. 3) compared with the TEL/PEL values. Furthermore, adverse biological effects will occur frequently at all zones in the dry season based on the level of naphthalene and at the Apapa zone in both seasons based on the level of naphthalene, acenaphthylene and acenaphthene (Fig. 3). The estimation of the probable effects quotient/factor (PELq) showed that slightly adverse effects will occur at all zones and in both seasons except at the Ilaje zone (wet and dry seasons) and Iddo zone (wet season) (SI 13). There is an 11% probability of toxicity to organisms (m-ERMq0.1) at all the zones and in both seasons based on the evaluation of the mean-effects range medium-quotient (m-ERM-q) (SI 14).
3.7. Pollution classification of sediments and fish species Sediments from all the zones and in both seasons were moderately polluted while sediments from the Apapa zone were highly polluted in the wet season based on the pollution classification of sum polycyclic aromatic hydrocarbons in sediments (Fig. 4). Fish species from all the zones and in both seasons were minimally contaminated except for S. melanotheron from the Ilaje zone which was not contaminated in both seasons based on the contamination classification of sum PAHs in fish species (Fig. 5).
3.8. Ranking of specific polycyclic aromatic hydrocarbons identified as ecological risk factors in the Lagos lagoon The ranking of specific PAHs identified as ecological risk factors in water, sediment and fish species from the Lagos lagoon showed that naphthalene and acenaphthene occurred in Three (3) (75%) out of the Four (4) ecological risk indices (SI 3G) specifying potential biological effects of specific PAHs (Fig. 6). The second most identified PAHs were acenaphthylene, fluorene, pyrene and benzo[a] anthracene occurring twice in the ecological risk indices. The least occurring PAHs identified as ecological risk factors were anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene and indeno[1,2,3-cd]pyrene (Fig. 6).
4. Discussion In this study, the DO and TDS values across the zones and seasons were lower and higher than the set limits respectively by the National Environmental Standards and Regulations Enforcement Agency (NESREA, 2010). The range of pH and temperature recorded in this study is similar to the values observed previously in the lagoon (Ayoola and Kuton, 2009). The salinity regime of the Lagos lagoon has been linked to the insignificant pH range observed in this environment. High water salinity in the dry season has been majorly attributed to low freshwater inputs and increased evaporation and vice versa during the wet season in the lagoon (Ayoola and Kuton, 2009). The dominance of naphthalene (2-ring PAHs) in the samples across the zones is consistent with the results of Asagbra et al. (2015) in the Warri River, Nigeria and Li et al. (2015) in the Yangpu Bay, China. Similarly, Sinaei and Mashinchian (2014) observed a dominance of LMW-PAHs in the coastal waters of the Persian Gulf. High LMW-PAHs level may be due to their solubility and persistence in the water column compared to the HMW-PAHs which are more hydrophobic, thus bound to sediments (Sinaei and P Mashinchian, 2014). The range of 16 PAHs in the surface waters is several folds higher than the values reported by Benson et al. (2014). The highest PAHs level in both seasons recorded at the Apapa zone could be due to the high volume of petroleum-related activities that are characteristic of the zone which has been indicated by various researchers (Doherty and Otitoloju, 2016; Sogbanmu et al., 2016). The preponderance of petroleum tank farms around this zone, high traffic of ships and boats utilizing petroleum products are potential cursors of the observed high PAHs level. The observed high levels of HMW-PAHs which include carcinogenic PAHs in the wet season compared to the dry season may be attributable to urban run-off of oil and atmospheric deposition, as well as, resuspension of HMW-PAHs that are adsorbed onto sediments into the water column during the wet season. It P is noteworthy that the high percentage composition of carc.PAHs in the HMW-PAHs profile points to the degree of potentially mutagenic, genotoxic and carcinogenic pollutants that are present in the Lagos lagoon water column (Adeogun et al., 2019). The dominance of the 2-ring PAHs (naphthalene) in the sediments is consistent with the results of Sojinu et al. (2013) who reported a similar trend in the sediments of the Ologe lagoon. However, Karacik et al. (2009) reported a dominance of 4e5 ring PAHs in the sediments of the Istanbul strait, Turkey. Sum carcinogenic PAHs, also known as toxic PAHs (Tavakoly Sany et al., 2014) in
Fig. 2. Estimated biological effects in Lagos lagoon sediments based on the values of polycyclic aromatic hydrocarbons compared to their respective Effects Range Low/Effects Range Medium (ERL/ERM) values, KEY: n ¼ 4. *represents significant difference between means at p < 0.05.
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Fig. 3. Estimated biological effects in Lagos lagoon sediments based on the values of polycyclic aromatic hydrocarbons compared to their respective Threshold Effect and Probable Effect levels (TEL/PEL), Key: ABEF - Adverse biological effects will occur frequently; ABEO - Adverse biological effects will occur occasionally.
Fig. 4. Pollution classification of Lagos lagoon sediments based on sum polycyclic aromatic hydrocarbons, Key: HPL e High Pollution Level, MPL e Moderate Pollution Level, LPL e Low Pollution Level.
the Lagos lagoon sediments were approximately 5 times higher than the range reported for the Ologe lagoon (Sojinu et al., 2013). P The percentage composition of the carc.PAHs in the sediments of this study is notable because they have greater resistance to microbial degradation (Yun et al., 2003). The highest PAH levels recorded at the Apapa zone in both seasons is consistent with the nature of anthropogenic activities at the zone as delineated in the P surface water and previous studies. However, the average PAHs in this study is lower than the values in the sediments of the Klang P strait, Turkey (Tavakoly Sany et al., 2014). The PAHs in the sediments from all the zones except Apapa were below the guideline of 1000 ng g1 (mg kg1) suggested by Johnson et al. (2002) for the protection of estuarine fish against deleterious health effects. The dominant fish species reported in this study are consistent with the findings of Ayoola and Kuton (2009) who recorded similar fish species in the Lagos lagoon. The predominance of LMW-PAHs and HMW-PAHs specifically, the 3- and 4-ring PAHs in the fish species across the zones is slightly similar to the dominant PAHs (LMW-PAHs) in the surface water and sediments. Similarly, a
dominance of HMW-PAHs was observed in fish species from GhaP naian coastal waters (Nyarko et al., 2011). The ranges of PAHs and P carc.PAHs in this study are lower than the findings of Nyan and Muralidharan (2012) in fish species from the harbour line, Mumbai, India. The ability of fish species to bioconcentrate and metabolise PAHs may depend on factors such as the route and duration of exposure, lipid content of tissues, environmental factors, species difference, age, sex and exposure to other xenobiotics (Varanasi et al., 1987). Most of the carcinogenic PAHs not detected in the fish species in this study may be due to their depuration or biotransformation (Deb et al., 2000). In this study, the observed high sediment to water-PAHs ratio indicates that PAHs in the sediments were higher than the levels in the water with certain PAHs like naphthalene, pyrene and benzo[k] fluoranthene reaching 2e3 folds in the sediment compared to the surface water in the wet season. This may be due to frequent episodes of accidental spillages at oil depots, operational shipping loss in the harbour, oil spills or leakages from ships and boats that traverse the lagoon, dredging activities in the lagoon and
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Fig. 5. Pollution classification of Lagos lagoon fish species based on sum polycyclic aromatic hydrocarbons. Key: MiC e Minimally Contaminated, NC e Not Contaminated.
Fig. 6. Ranking of specific polycyclic aromatic hydrocarbons identified as ecological risk factors in the Lagos lagoon, Key: ERI e ecological risk indices, Naph e naphthalene, Acenaph e acenaphthene, Acenaphthy e acenaphthylene, Fluo e fluorene, Pyr e pyrene, BaA e benzo[a]anthracene, An e anthracene, Chry e chrysene, BbF e benzo[b]fluoranthene, BkF e benzo[k]fluoranthene, IdP e indeno[1,2,3-cd]pyrene.
contributions from effluents that are discharged into the lagoon (Adeogun et al., 2019). The significantly high sediment to waterPAHs ratio for pyrene especially in the wet season reflects the contamination of the sediments with HMW-PAHs. Pyrene represents a major portion of the total PAHs found in contaminated sites (Luo et al., 2006) and has been used as an indicator of PAHs contamination in the monitoring of wastes (Dissanayake and Bamber, 2010). The wet season with its attendant run-off of storm water, atmospheric deposition into the water column is implicated for the deposition of this PAH into the sediments. The observation of the highest BCF of 2.28 in G. melanopterus associated with pyrene is comparable with the highest sediment to waterPAHs ratio observed for pyrene. The lipophilic nature of pyrene as a HMW-PAH, its ubiquity and persistence could promote its bioaccessibility and consequent bioavailability in the fish tissues. In this study, the Iddo zone was not as contaminated with PAHs as observed for Atlas Cove and Apapa zones. Thus, the high BCF factor for pyrene could be related to the specific nature of the dominant
fish at the zone which is different from the dominant species at the other zones. Conversely, the low BCF (<1) for the fish species from other zones and PAHs could be an indication of a minimal contamination level of PAHs in the fish species. The estimation of frequent biological effects in organisms from the Apapa zone based on the TEL/PEL values is consistent with findings of Sogbanmu et al. (2016) who reported significant teratogenic and genotoxic effects in Danio rerio exposed to sediment extracts from the zone. It is noteworthy that the specific PAHs identified as the cursors of potential biological effects in the sediments are mainly LMW-PAHs that are known to be acutely toxic (Asagbra et al., 2015) and have been identified as ecological risk factors in sediments from the Yellow River, China (Feng et al., 2016). Occasional biological effects estimated for organisms at the Atlas Cove zone may be due to the sand dredging and petroleum discharge activities peculiar to the zone. The biological effects to organisms established based on the ecological risk indices in this study corroborate observations by various authors (Amaeze et al.,
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2015; Sogbanmu et al., 2016; Adeogun et al., 2019). Although, slight adverse effects and 11% probability of toxicity was estimated for organisms across the zones based on the PELq and m-ERM-q, the potential ecological risks posed by the sediments vis-a-vis the weight of evidence shown in the PAH levels, nature of anthropogenic activities and report of previous studies points to the need to take decisive actions in order to protect and conserve the aquatic organisms and man who are consumers of fisheries from this coastal ecosystem. The classification of sediments as moderately polluted, particularly the Apapa zone which was classified as highly polluted is similar to findings in coastal sediments of China (Li et al., 2007) and Mason Bay, Korea (Yim et al., 2005). Furthermore, this result corroborates the estimation of toxicity of sediments in this study which identified frequent biological effects at the Apapa zone and occasional biological effects at the other zones and in both seasons. The minimal contamination of fish species in this study is consistent with the low bioconcentration factor values (less than 1) observed in the fish species. This may be attributed to the low lipid content of the scaled fish species (Abdel-Shafy and Mansour, 2016). Fishes tend to have lower PAH concentrations in their body tissues compared to the water in which they live due to their ability to metabolise and excrete the compounds (Johnson et al., 2002; Johnson-Restrepo et al., 2008). However, S. melanotheron at the Ilaje zone was classified as not contaminated which could be attributed to the low levels of PAHs observed at the zone. The PAHs identified as ecological risk factors in the Lagos lagoon especially naphthalene and acenaphthene are associated with various industrial activities apart from petroleum-related activities. Acenaphthene is utilized in the production of pigments, dyes, pesticides, pharmaceuticals among other uses (Abdel-Shafy and Mansour, 2016). The various industries and residential houses which discharge their effluents (either treated or untreated) into the lagoon might be relevant sources of this PAHs in the lagoon. It is noteworthy that the PAHs are risk factors in both wet and dry seasons except in some instances where specific zones and seasons are identified (SI 3G).
5. Conclusions This study has delineated the levels of PAHs in three matrices of four anthropogenic-impacted zones of the Lagos lagoon. The high PAHs content and risk to aquatic organisms based on the ecological risk indices at the Apapa zone are reminiscent of the nature of activities (mainly shipping and port activities) at the zone which are point sources of hydrocarbons input into the lagoon. A robust ecological risk assessment has been done perhaps for the first time in the Lagos lagoon which revealed the moderate to high pollution level of the sediments in particular. Specific PAHs such as naphthalene, acenaphthene, acenaphthylene, fluorene, pyrene and benzo[a]anthracene have been identified to pose major ecological risks to organisms in lagoon. We recommend their inclusion in ecomonitoring programmes based on the UN sustainable development goal 14 (life below water). Also, the ecological risk assessment studies have revealed the need for a review and/or setting of guidelines for individual and sum PAHs in coastal waters, sediments and fish for sustainability of aquatic ecosystems and safeguard of public health through fish consumption.
Conflicts of interest The authors declare no conflicts of interest.
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